Referring to FIG. 6a, a view of one exemplary configuration of a semiconductor laser module is shown in cross section. In FIG. 6b is shown an example of electric wiring diagram of the semiconductor laser module of FIG. 6a. The semiconductor laser module 1 is such that a semiconductor laser element 2 and an optical fiber 3 are optically coupled together forming a module.
A thermo-module 5 is provided on the inner bottom wall plane 4a of a package 4. The thermo-module 5 is constructed in such a form that a plurality of Peltier elements 5a are placed between plate member 5b (first substrate, first board) and plate member 5c (second substrate, second board) which are insulation substrates made of, for example, alumina (aluminum oxide), aluminum nitride, or another suitable material. In this example, the plate member 5b is fixed on the inner bottom wall plane 4a of the package 4, and the heat radiation side of the Peltier elements 5a are secured on the plate member 5b by soldering, and the plate member 5c is secured on the heat absorption side of the Peltier elements 5a by soldering.
Such a thermo-module 5 changes the heat emission action (heating action) and the heat absorption action (cooling action) in response to orientation of a current which flows in the Peltier elements 5a, wherein the heat emission quantity and heat absorption quantity vary in response to the amount of current flow within the Peltier elements 5a. 
A substrate 6 which is a member for attaching components is installed and fixed on the upper side of the thermo-module 5 (the plate member 5c) by solder consisting of, for example, InPbAg eutectic solder having a melting point of 148° C. Supporting members 7 and 8, and a lens 9 are fixed on the upper side of the substrate 6. On the supporting member 7, the semiconductor laser element 2 is disposed, and a thermister 10 for detecting the temperature of the semiconductor laser element 2 is provided. On the supporting member 8, a photo diode 11 is disposed for monitoring a light emitting state of the semiconductor laser element 2. Typically, a laser element having signal light wavelength bands of, for example, 1310 nm and 1550 nm, and wavelength bands of pumping light for optical fiber amplifiers such as a 1480 nm band and a 980 nm band, etc. is used as the semiconductor laser element 2.
A through hole 4c is provided at the side wall 4b of the package 4, and an optical fiber supporting member 12 is fitted in the through hole 4c and mounted therein. The optical fiber supporting member 12 has an insertion hole 12a, wherein an end portion of an optical fiber 3 is introduced from the outside of the package 4 through the insertion hole 12a. Also, a lens 14 is disposed inside the insertion hole 12a with a spacing between it and the end portion of the optical fiber 3.
As shown in FIG. 6b, a plurality of lead pins 16 (14 pins in the example shown in FIG. 6b) are formed at the package 4 so as to protrude upward therefrom. In addition, electrical couplings 17 such as conductor patterns and lead wires, etc. are provided in the package 4 to electrically connect the semiconductor laser element 2, thermo-module 5, thermister 10, and photo diode 11 to the abovementioned lead pins 16. By the electrical couplings 17 and lead pins 16, the semiconductor laser element 2, thermo-module 5, thermister 10 and photo diode 11 can, respectively, be electrically connected to a drive controller (not illustrated) for driving the semiconductor laser module.
In detail, in the example shown in FIG. 6b, the semiconductor laser element 2 is electrically connected to the drive controller by the electrical coupling 17 and lead pins 16 (16g and 16h), the thermo-module 5 by the electrical coupling 17 and lead pins 16 (16a and 16f), the thermister 10 by the electrical coupling 17 and lead pins 16 (16b and 16e), and the photo diode 11 by the electrical coupling 17 and lead pins (16c and 16d).
A semiconductor laser module 1 shown in FIG. 6 is constructed as described above. When such a semiconductor laser module 1 is electrically connected to the drive controller and current flows from the drive controller to the semiconductor laser element 2 of the semiconductor laser module 1, laser light is emitted from the semiconductor laser element 2. The emitted laser light is condensed by an optical coupling system comprising the lenses 9 and 14 and is permitted to enter the optical fiber 3, wherein the light propagates through the optical fiber 3 and is used for appointed applications.
However, the intensity and wavelength of laser light emitted from the semiconductor laser element 2 are known to fluctuate in response to the temperature of the semiconductor laser element 2 itself. Therefore, in order to maintain the intensity and wavelength of the laser light constant, the drive controller controls the direction of current flow within the thermo-module 5 and the amount of current flowing therein on the basis of an output value provided from the thermister 10, thereby controlling the heating action and cooling action of the thermo-module 5. Through control by the thermo-module 5, the semiconductor laser element 2 is typically kept at an almost constant temperature, whereby the intensity and wavelength of the laser light emitted from the semiconductor laser element 2 is constant.
By an erroneous operation or an overvoltage, however, there may be caused an abnormal situation in which an overcurrent is fed to the thermo-module 5 in a direction for causing the thermo-module 5 to heat. In this case, the thermo-module 5 is extraordinarily heated so abruptly causing components such as the semiconductor laser element 2, the substrate 6 or the lens 9, as arranged on the thermo-module 5 to be heated such that the indicated temperature of the thermistor 10 rises to 200° C. or higher within 10 seconds.
When the plate member 5c of the thermo-module 5 is thermally connected to the side wall of the package 4 and the optical fiber supporting member 12, a portion of the heat emitted from the thermo-module 5 is discharged from the module via the side wall of the package 4 and the optical fiber supporting member 12. Therefore, when the thermo-module 5 is extraordinarily heated as described above, the amount of heat transmitted to components on the thermo-module 5 such as the semiconductor laser element 2, lens 9, etc., is suppressed since heat is discharged from the thermo-module 5 thereby relieving some of the temperature increase for the components on the thermo-module 5.
But, in the configuration of FIG. 6, the components on the thermo-module 5 are thermally isolated from the sidewall of the package and the optical fiber supporting member 12. Therefore, little of the heat is quickly dissipated through the sidewall of the package 4. In such a case, when the thermo-module 5 is extraordinarily heated to a high temperature, the high temperature heat of the thermo-module 5 is transmitted to the components on the thermo-module 5 and is accumulated there. Accordingly, the temperature rise of the components on the thermo-module 5 is substantial, and the following problematic situations may arise.
For example, as described above, when the temperature of the semiconductor laser element 2 is raised to a high temperature due to rapid heating of the thermo-module 5 resulting from an overcurrent flowing therein in the heating direction, known problems may occur, where a defect in crystal of the semiconductor laser element 2 grows and the characteristics of the semiconductor laser element 2 deteriorate to a large extent.
In addition, as described above, the substrate 6 is fixed on the plate member 5c of the thermo-module 5 by thermal-fusion type connection material, for example, solder such as InPbAg eutectic solder having a melting point of 148° C. For this reason, where the thermo-module 5 is heated to an extraordinarily high temperature, the solder is melted to cause the substrate 6 to be shifted from its original, properly aligned fixed position. The positional shift of the substrate 6 causes the semiconductor laser element 2 and the lens 9 to shift from their originally aligned positions, whereby problems occur such as optical decoupling. The misalignment causes the semiconductor laser element 2 and lens 9 to shift with respect to the optical fiber 3. In particular, if an angular shift of 0.2° occurs in the semiconductor laser element 2 with respect to the optical fiber 3, a 95% loss in optical output may result causing substantial lowering of the optical output intensity.
Further, the glass-made lens 9 is adhered to, for example, a metal-made holder, utilizing glass solder which is fixed at the substrate 6 in order to fix the lens 9 to the substrate 6. In this case, as described above, when the thermo-module 5 is quickly overheated, a crack occurs at the junction point between glass and solder between the lens 9 and the metal holder by a large difference in the thermal expansion ratio between glass and metal. Problems arise, wherein, by occurrence of the crack, the lens 9 dislodges from the metal holder, and the optical coupling between the semiconductor laser element 2 and the optical fiber 3 may be disrupted.
Still further, as described above, since the Peltier element 5a, and plate members 5b and 5c are fixed together with solder, rapid heating causes melting of the solder, whereby, for example, the Peltier element 5a comes off, and the thermo-module 5 itself may be damaged.
The invention was developed to solve the above mentioned problems, and it is therefore an object of the invention to provide a semiconductor laser module that can prevent overcurrent and overvoltage within the thermo-module in the heating direction thereby avoiding problems resulting from overcurrent. It is also an object of the invention to provide a method for driving the semiconductor laser module.